1. Field of the Invention
The present invention is related to the fabrication of bipolar supercapacitor that is spirally wound with edge sealing, and in particular to the fabrication of supercapacitor modules comprised of a plural number of supercapacitor rolls disposed in the compartments of a container to provide powers of high voltages and high currents.
2. Description of the Related Art
Battery is the most commonly used portable device of energy storage and energy supply. In the use of battery, two functional criteria of the device, that is, use-time and power output determine the practicality of battery, especially in vehicular applications. For attaining a long use-time, the electrodes of battery are generally in a large mass, otherwise, a new electrode material must be developed and verified as in the battery evolution from lead-acid to lithium batteries. Contrary to the thick-electrode strategy, thin electrodes are employed for battery to deliver high power output as taught in U.S. Pat. Nos. 5,047,300; 5,108,848; 5,223,351 and 5,993,983. In the first three patents of the afore-listed, both thick and thin electrodes are incorporated within the same battery case. Battery of thin metal film is utilized in patent ""983 for being attached to an existing battery as a power booster. Similar attached-type of power assistant is revealed in U.S. Pat. Nos. 5,568,537; 5,637,978 and 5,796,188. Charge and discharge of battery have always involved chemical reactions, consequently, the power output of battery is inherently limited by the reaction rates. In terms of response time, chemical reactions are slower than physical processes. Therefore, the power output of battery is inferior to that of capacitor for the latter depending on physical process such as charge accumulation or surface adsorption for energy storage. Capacitor is logically a better booster than thin-electrode batteries to the thick-electrode batteries, and the use-time of the latter can be prolonged as well.
Similar to the power density is a deficiency of battery, capacitor has its own shortfall in energy density. Ironically, it is the same physical process imparting capacitor high power density that causes low energy density for the capacitor. It may be described as xe2x80x9ceasy come easy goxe2x80x9d. The energy content of capacitor can be calculated by equation (1)
xe2x80x83E=(xc2xd)CV2xe2x80x83xe2x80x83(1)
where E is the stored energy in joule (J), C is capacitance in farad (F), and V is working voltage of capacitor in volt (V). Apparently, by increasing both the values of C and V, the energy content of capacitors is augmented. Because of the power of 2, V has a larger effect then C on improving the energy density of capacitor. Next, the capacitance value (C) of capacitor can be calculated through equation (2). Equation (2) depicts that C is proportional to the dielectric constant (K) of electrode material of capacitor and the electrode area (A) in square meter, and inversely proportional to the distance between electrodes (D) in meter.
xe2x80x83C=KA/Dxe2x80x83xe2x80x83(2)
In order to enhance the value of C, electrode materials with high K values are developed, and the surface area of electrode (A) is enlarged by etching or other means, while the spacing between the electrode (D) is minimized through tight packaging of the electrodes. Supercapacitor is one kind of electrochemical capacitor that utilizes materials with tremendous surface area such as carbonaceous material, or a metal oxide with catalytic activity to store electric energy via physical processes at the interface of electrode and electrolyte. Besides supercapacitor, there are other names invented to describe capacitors with large capacitance values, for example, the device used carbonaceous material is called electric double layer capacitor (EDLC), while metal-oxide one is ultracapacitor. Given large surface area, supercapacitor can store static charge up to several thousands of farad.
Though supercapacitor may contain a large value of capacitance, the voltage built across two electrodes, that is, the anode and the cathode, ranges only from 1.0 V to 3.0 V depending on whether an aqueous solution or an organic solution is employed as the electrolyte for the capacitor. As a matter of fact, the low working voltages of supercapacitor correspond to the decomposition voltages of the solvents of the electrolyte used for the capacitor. Generally, the organic electrolytes offer higher working voltages than the aqueous counterpart at the expense of conductivity. While new electrolyte systems are being widely developed to elevate their withstanding voltages, the working voltage of supercapacitor is conveniently and instantly promoted through series connections. There are two ways to achieve the series connection for supercapacitor, one of them is to integrate plural electrodes into a single capacitor as disclosed in U.S. Pat. Nos. 5,450,279; 5,955,215; 6,005,764; 6,187,061; 6,449,139 and 6,507,479, the other is to assemble plural individual capacitor units after encapsulation into a power module as revealed in U.S. Pat. Nos. 6,072,691 and 6,215,278. All of the U.S. Patents cited above are incorporated herein by reference. In-cell connection of plural electrodes presents several advantages over the series connection of multiple encapsulated units: 1) only one casing is employed versus plural containers for the latter, 2) short cable, or no connecting cable in some cases, versus long electrical cables for the latter, 3) one-time encapsulation versus multiple operations for the latter, and 4) homogenized internal resistance or equivalent series resistance (ESR) versus diversified ESRs among the units of the latter. For in-cell series connection of the electrodes, an effective and economic method is the utilization of bipolar electrode. The forgoing electrode is an electrode that can serve simultaneously as positive and negative electrodes. In other words, one side of the bipolar electrode carries positive polarity and the other side negative. Structurally, the bipolar electrode is created by confining electrolyte within two electrodes of a unit cell with no communication of electrolyte between cells. Henceforth, the minimum number of electrodes of a bipolar packaging is three, wherein the first and third electrodes are anode and cathode, respectively, and the middle one is the bipolar electrode. Using the bipolar design, two unit cells are connected in series without connecting wire to form a single package, and less material is consumed to attain the same operation voltage as that provided from series connection of two encapsulated capacitor individuals. This will be elaborated in the detailed description of the invention.
Two prior arts, for example, U.S. Pat. Nos. 5,450,279 and 6,005,764, have utilized the bipolar design to assemble their capacitors. Nevertheless, both works rely on stacking of electrodes to construct the capacitors. Patent ""279 also employs separator bag, embedded separator and embedded current collector for the bipolar effect, whereas patent ""764 uses painstaking piling of numerous unit cells, for example, 100 pairs of anode and cathode for a working voltage of 100V. Assembly of cells by the stacking approach requires many processing steps at manufacturing, which will reduce the throughput while increase the cost. Furthermore, after the electrodes are stacked, the whole stack is often secured by two end plates in conjunction with bolts and nuts resulting in a bulky and heavy device. Supercapacitor should be fabricated through easy preparation of the unitary cell, followed by simple assembly and packaging to meet the application needs. When the cost of supercapacitor can be controlled by simple and productive fabrication, together with just-enough use of material, the capacitor may be widely accepted.
Supercapacitor is a versatile energy-storage device, yet it receives incommensurate attention in the industries. Unit price and electrical performance are the two major causes that prevent supercapacitor from prevalence. Accordingly, an object of the present invention is to fabricate supercapacitor with sufficient energy density in a single package to fulfill application requirements. Integration of the required number of unit cells into one piece of the desired device at the fabrication stage involves lower cost and less hustle than post conjunction of encapsulated capacitor individuals. Furthermore, such integration will offer end users a lot of convenience.
Another object of the present invention is to use spiral winding of as needed electrodes and separators into a cylindrical, oval or square roil of unitary supercapacitor. Each unit capacitor is prepared in the right size to yield sufficient capacitance as demanded by applications, as well as in an adequate configuration for the highest space utilization of the capacitor housing. Comparing to stacking, winding can generate high surface area for the unitary supercapacitor in one step of operation. Using automatic machineries, the winding operation can produce supercapacitor at a high throughput so that the products have commercial viability.
Still another object of the present invention is to employ bipolar packaging for promoting the working voltage of unitary supercapacitor. There are four applications of edge sealing at four different stages of spiral winding of the electrodes and separators using a curable polymer. Not only the edge sealing can confine the electrolytes within two electrodes of every unit cell, but also it can insulate the electrodes to prevent electric shorts or electrochemical reactions, so that the leakage current (LC) of capacitor may be reduced. As the electrodes are fixed through edge sealing, convulsion damage of the electrodes at charging and discharging of the capacitor may be minimized.
Yet another object of the invention is to place as needed number of unitary supercapacitors in a container with as needed number of compartments so that the individual units can be integrated into a single device with sufficient energy density. When the power load of an application is determined, the number, as well as the dimension, of the compartments of capacitor housing can then be decided. Correspondingly, the unitary supercapacitors are fabricated and assembled in the container to meet the application need without undue wasting of material. There is no restriction on the shape of container to house the unitary supercapacitors as long as the integrated supercapacitor can fit the space of application.
Still yet another object of the invention is to employ a curable potting material to hermetically seal the capacitor housing to complete the integration of unitary supercapacitors. In conjunction with the edge sealing, potting should collectively and effectively assist the electrodes against convulsion damages from mechanical movements induced during charge and discharge of the supercapacitors. Many potting materials can provide a mechanical strength comparable to that offered by bolts and nuts. Nevertheless, the potting operation is easier and quicker than the screwing, and the former also presents a permanent fastening.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.